Relationship between coral distributions and physical variables in Amitori Bay, Iriomote Island, Japan

Abstract

The relationship between coral distributions and physical variables was investigated in Amitori Bay, Iriomote Island, Japan. Field observations were conducted to obtain data on coral distributions, sea temperature, sea salinity, wind speed, and river flow rate. The observed data were then used in ocean and wave model numerical simulations and soil particle tracking analysis to obtain the spatial and temporal distributions of wave height and the numbers of soil particles. The main results of this study indicate that wave height and the number of soil particles have a significant correlation with coral distribution. Higher wave heights result in greater coverage of tabular coral and lower coverage of branching coral. A greater number of soil particles relates to lesser coverage of tabular coral. On the contrary, the number of soil particles is not correlated with branching coral coverage. The potential habitats for tabular corals largely depend on the species. Acropora hyacinthus is distributed in conditions with low numbers of soil particles, A. digitifera is distributed in conditions with low numbers of soil particles and high wave heights, and A. millepora is distributed in conditions with high numbers of soil particles and low wave heights. Averages of diversity index of the coral types at the mouth and inner parts of the bay are lower than average of the whole region, but average of diversity index at the intermediate part of the bay with the intermediate physical disturbances is higher than it, which seems to support the intermediate disturbance hypothesis.

Transport properties of soil particles in Sakiyamawan–Amitoriwan nature conservation area, Iriomote Island, Japan

“…Therefore, the CCM is suitable for calculating ocean currents in regions such as the Sakiyama and Amitori bays, which have a large bathymetric gradient ranging from a few meters depth for the coral reefs to 70-m depth for the ocean bottom floor. In fact, previous studies showed that the CCM can reproduce the observations with high precision in Amitori bay (Shimokawa et al 2014b; see Appendix 2) and also other areas (Murakami et al 2015c). In addition, the CCM can support accurate calculation of the inflow rates of freshwater Fig.…”

“…These features are explainable mainly by seasonal differences in wind speed and direction, but the combination among seasonal differences in wind speed and direction, wind-driven current and topography is also important for them. For Amitori bay, we have conducted studies on the soil particle impact on coastal marine ecosystems such as corals (e.g., Murakami et al 2012;Shimokawa et al 2014bShimokawa et al , 2015Shimokawa et al , 2016Ukai et al 2015) and E. acoroides (e.g., Murakami et al 2014Murakami et al , 2015aNakase et al 2015Nakase et al , 2016. Those results provide the basic information about whether the control of soil input from the rivers should be done or not and how much it should be done.…”

“…Therefore, the critical Shields number is set to a constant value (=0.05), which is used in inner bay for previous study (Kawasaki et al 2008). The mass percentage passing of soil particles from the Ayanda and Udara rivers obtained by sediment trap observation at the mouths of the rivers (Shimokawa et al 2014b) showed that the diameters of the soil particles were distributed mainly from 0.1 to 50 μm. This study focuses on the distributions of soil particles.…”

“…In Amitori bay (Shimokawa et al 2014b), regions 8, 9, and 10 are deep (maximum 70 m) with little corals and no E. acoroides. E. acoroides existed formerly in region 6, east part of region 7, and region 15, but they decrease at preset (Nakase et al , 2016.…”

“…Previous studies have shown that the distributions of reef-building corals (Murakami et al 2012;Shimokawa et al 2014bShimokawa et al , 2015Shimokawa et al , 2016Ukai et al 2015) and E. acoroides (Murakami et al 2014(Murakami et al , 2015aNakase et al 2015Nakase et al , 2016 are largely influenced by the environmental gradients and vary with location in the Sakiyama and Amitori bays. However, the impact of soil input from rivers on the coastal marine ecosystems has not been investigated thoroughly in the area, especially in Sakiyama bay.…”